PostgreSQL is a very powerful open source database management system. It's licensing is clear in that you can use if for whatever you like. That includes using it in a commercial product.
What we like about PostgreSQL:
- No licensing issues.
- Standards based.
- Highly scalable.
- Fast and powerful. Can handle billions of records.
- Publicly available support is good.
What we don't like aboiut PostgreSQL:
- Is challenging to install, update, and maintain.
For more information on PostgreSQL visit the webpage here: http://www.postgresql.org/
For a comparison with MySQL go here: http://www.wikivs.com/wiki/MySQL_vs_PostgreSQL